Beecroft Institute for Particle Astrophysics and Cosmology

Abstract:

ABSTRACT In chameleon f(R)-gravity, the fifth force will lead to ‘upturns’ in galaxy rotation curves near the screening radius. The location of the upturn depends on the cosmic background value of the scalar field $\bar{f}_\mathrm{R0}$, as well as the mass, size, and environment of the galaxy. We search for this signature of modified gravity in the SPARC sample of measured rotation curves, using an MCMC technique to derive constraints on $\bar{f}_\mathrm{R0}$. Assuming NFW dark matter haloes and with $\bar{f}_\mathrm{R0}$ freely varying for each galaxy, most galaxies prefer f(R) gravity to ΛCDM, but there is a large spread of inferred $\bar{f}_\mathrm{R0}$ values, inconsistent with a single global value. Requiring instead a consistent $\bar{f}_\mathrm{R0}$ value for the whole sample, models with $\log _{10}|\bar{f}_\mathrm{R0}|\gt -6.1$ are excluded. On the other hand, models in the range $-7.5\lt \log _{10}|\bar{f}_\mathrm{R0}|\lt -6.5$ seem to be favoured with respect to ΛCDM, with a significant peak at −7. However, this signal is largely a result of galaxies for which the f(R) signal is degenerate with the core/cusp problem, and when the NFW profile is replaced with a cored halo profile, ΛCDM gives better fits than any given f(R) model. Thus, we find no convincing evidence of f(R) gravity down to the level of $|\bar{f}_\mathrm{R0}|\sim 6 \times 10^{-8}$, with the caveat that if cored halo density profiles cannot ultimately be explained within ΛCDM, a screened modified gravity theory could possibly provide an alternative solution for the core/cusp problem. However, the f(R) models studied here fall short of achieving this.

Abstract:

Long-lived pseudo-solitonic objects, known as oscillons/oscillatons, which we collectively call real scalar stars, are ubiquitous in early Universe cosmology of scalar field theories. Typical examples are axions stars and moduli stars. Using numerical simulations in full general relativity to include the effects of gravity, we study the fate of real scalar stars and find that depending on the scalar potential they are either meta-stable or collapse to black holes. In particular we find that for KKLT potentials the configurations are meta-stable despite the asymmetry of the potential, consistently with the results from lattice simulations that do not include gravitational effects. For α-attractor potentials collapse to black holes is possible in a region of the parameter space where scalar stars would instead seem to be meta-stable or even disperse without including gravity. Each case gives rise to different cosmological implications which may affect the stochastic spectrum of gravitational waves.